The invention relates to a method for the manufacture of glass bodies wherein a porous green body is formed by extrusion from the starting material for the glass body in the form of a gel and this green body is then purified and sintered.
The invention also relates to equipment for implementing such a method and applications of the glass bodies produced by the method according to the invention.
The method referred to in the preamble is particularly suitable for the production of rods and especially tubes of fused silica, e.g. as preforms for optical waveguides.
Optical waveguides are used for a variety of applications, either for short-distance or long-distance light-transmission systems as in the case of optical communications systems and consist mainly of a glass of high silica content (which, if necessary, contains a doping agent for adjusting the refractive index of the glass).
The British Patent No. GB 10 10 702 describes a method in which pure or practically pure powdered SiO.sub.2 with a liquid binder in a quantity of 1 to 50 wt. % and, where appropriate, a further lubricant to promote the extrusion process, in a quantity of 0.1 to 10 wt. % per SiO.sub.2 content is worked into an extrusion mass and formed by an extrusion process. The liquid binders which may be used are, for example, organic liquids such as polyvinyl alcohol or water. This method corresponds to the ceramic technique in which ground, powdered raw materials with grain diameters of &gt;1 .mu.m are processed with water, binders and lubricants into high-viscosity extrudable gels.
In the processing of highly disperse starting powders with grain diameters of &lt;1 .mu.m (which can not be produced by conventional grinding processes), such as are used for the manufacture of fused silica bodies (particularly for preforms of optical waveguides) problems arise with the mixing and/or kneading particularly when the starting compounds contain a high propotion of disperse phase because then a much large number of particles have to be uniformly distributed and a correspondingly large surface has to be uniformly covered with the additives (binders and lubricants). Thus, for example, the typical mixing and kneading times for a starting mass of highly disperse fused silica particles (10 to 100 nm particle diameter) with 60 wt. % SiO.sub.2 and 36 wt. % water (residue of additives) are one to three hours. In addition to this substantial time requirement for homogenization of an extrudable starting mass, there is a further problem: The gels to be processed, with SiO.sub.2 powders as the main constituent and water as the dispersing agent, prove to be extremely complex formations rheologically, and it is practically impossible to predict how they will behave in the forming process.
It has been shown that even slight differences in composition and pretreatment and storage of the gels have extremely marked influences on the behaviour during extrusion. This will be illustrated here with the example of two gels of almost identical composition and treatment (cf Table I):
TABLE I ______________________________________ Composition Mixture 1 Mixture 2 ______________________________________ 1. Highly disperse SiO.sub.2 400 g 400 g with a particle size in the range from 10 to 500 nm 2. Water (dispersing agent) 224 g 222 g (solids:water = 1:2) 3. Wetting agent 1.0 g 0.8 g (e.g. polyvinylalcohol) 4. Lubricant 25.0 g 25.0 g (e.g. butylstearate) 5. Kneading and homogeni- 4.0 h 3 h 35 min zation time t 6. Total mass M 650 g 647.9 g 7. Product M .multidot. t = K* 2600 2332 ______________________________________ *The Figure K = M .multidot. t is an arbitrarily selected quantity which designates the degree of homogenization.
Despite this almost identical composition and (except for the kneading time t) almost identical pretreatment there were found to be considerable differences during the extrusion. The data given in Table II below relate to a cold extrusion from the two mixtures of approximately 20 cm in length and 1.6 cm diameter using state-of-the-art extrusion equipment.
TABLE II ______________________________________ Parameters Mixture 1 Mixture 2 ______________________________________ Exit force P.sub.o 247 N 7002 N End force P.sub.1 2505 N 25000 N Feed rate of blank .sup.-.upsilon. 1.9 cm/s 1.0 cm/s ______________________________________
In Table II, P.sub.o signifies the minimum force applied resulting in the exit of the blank, P.sub.1 is the maximum force required for quantitative extrusion of the blank. During the extrusion (in the time t=10 s or t=20 s) the pressure had to be increased continuously.
This considerably differing behaviour of the two gels indicates that the flow or deformation behaviour corresponds to that of a so-called intrinsically viscuous medium. The reversible and irreversible processes in gels of the above-described type, associated with the Ostwald definition of intrinsic viscosity, are responsible for the problems occurring during the extrusion.